Transducers which translate non-electrical quantities into electrical signals are designed to provide a usable output in response to a particular physical quantity, property or condition which is desired to be measured. The term "sense element" has been used to define both the overall transducer as well as the transducer element which performs the first step in a multi-step translation process. For the purposes of the instant invention, the latter definition will govern.
Transducers with sense elements used to measure acceleration are known as accelerometers. Accelerometer applications in the automotive industry include incorporation into crash sensors for air bag deployment and ride motion sensors for active suspension components.
Recently, accelerometer sense elements have been fabricated in part from silicon by adaptation of conventional integrated circuit processing methods. For example, U.S. Pat. No. 4,483,194 discloses an acceleration sensitive element consisting of a flap suspended at two adjacent corners by means of an integrally processed torsion bar which maintains the flap, when at rest or at constant velocity, a fixed distance above a conductive electrode deposited on a planar glass substrate, forming a capacitor between the flap and the electrode. Upon acceleration normal to the plane of the device, the flap rotates about the torsion bar axis, changing the capacitance between the flap and the electrode. The change in capacitance may be compared to a standard capacitance in a bridge-type circuit. In either case, variation in temperature and changes in capacitance associated with aging of components render the device of U.S. Pat. No. 4,483,194 unsuitable for many exacting applications. Moreover, the change in capacitance is non-linear with respect to acceleration, and thus the output signal will not accurately track acceleration.
U.S. Pat. No. 4,736,629, hereby incorporated by reference, discloses an accelerometer employing a sense element which consists of a metallic upper plate having an internal opening surrounding a pedestal mounted to a semiconductor substrate. The pedestal is connected to the metallic plate by a pair of torsion members extending in opposite directions from the pedestal to the metallic plate. Fixed plates positioned on the semiconductor substrate correspond to portions of the metallic plate to form first and second capacitors. The position of the torsion arms and/or geometry of the metallic plates ensures that the portions of the plate on either side of the torsion bar axis have unequal moments. In response to acceleration normal to the substrate the metallic plate rotates around the flexure axis defined by the torsion members to vary the capacitance of the first and second capacitors.
Devices such as those disclosed in U.S. Pat. No. 4,736,629 represent an improvement over the devices of the '194 patent, as the capacitors on either side of the torsion bar axis are constructed of the same material and should thus be similarly affected by temperature, aging of components, and the like. Moreover, as the capacitance of one set of plates increases while the other decreases, sensitivity of the accelerometer is enhanced as opposed to comparing but a single capacitance to a standard, and the output has substantially improved linearity. The '629 device is not easily manufactured by semiconductor processing techniques, however, and being highly damped, is not sensitive to a wide range of accelerations.
U.S. Pat. No. 5,220,835 discloses a capacitance accelerometer sense element having a dielectric substrate and a semiconductive upper plate in which the torsion beams which define the flexure axis of the upper plate are themselves attached to an annular ring connected to a supporting pedestal by a pair of beams in the plane of the torsion beams but perpendicular to the axis of the torsion beams. This arrangement places the torsion beams under tension rather than compression when subject to thermal stress, reducing the potential of the torsion beams to deflect or buckle under such conditions, improving the linearity of the device as well as decreasing the chance of deflection where the upper plate comes in contact with the lower plate electrode, shorting the capacitor formed between the upper and lower plates.
In copending U.S. application Ser. No. 08/043,671 is disclosed a torsion bar accelerometer which improves notably on the performance of such devices. In the device disclosed in the '671 application and illustrated herein by FIG. 7, the upper plate of the accelerometer is a monolithic boron-doped silicon substrate having an internal opening in the upper plate adapted to contain a pedestal to which the plate is connected by torsion arms. The upper plate, torsion arms, and pedestal all are formed from the same silicon wafer by processes adapted from integrated circuit processing technology, and thus are economical to manufacture. To overcome the limitations with regard to wide frequency response, the upper plate contains numerous through-holes allowing a passage for air to escape or enter the area between the plates as they rotate about the torsion bar axis responsive to acceleration normal to the plane of the device. The '671 device therefore has a considerably enhanced range of response as compared to prior devices.
In the '629, '671, and '835 devices, however, symmetry in geometry of the two capacitors, needed to eliminate offsets and provide linear operation, is partially destroyed by the fact that the surface areas of the movable plates, or "tilt plates", which form the upper plates of the two capacitors are not identical. In operation, the charge induced on the electrodes on the planar substrate during operation may migrate to the area surrounding the electrodes. Even when the substrate is glass, normally considered an insulator, the limited conductance allows such charge spreading, particularly at elevated temperatures. The arm of the device of higher moment and thus larger plate area thus has an electrical geometry different from and larger than that of the opposite arm, even though the physical geometries of the conductive electrode surfaces may be identical. The difference in effective areas of the two capacitors increases nonlinearity of the response. In addition, charge spreading will increase the electrostatic attractive force between the upper and lower plates of the capacitor, tilting the plate and cause a drift in output. Moreover, the numerous through-holes in the semiconductive upper plate of the '671 device lowers the effective surface area of the plates. As the capacitance changes being measured are on the order of femtofarads, these defects are of considerable importance in accurately measuring acceleration.
It is thus an object of the present invention to provide an accelerometer sense element wherein the electrical geometries of the "heavy side" and "light side" capacitors are substantially similar.
It is a further object of the present invention to provide an accelerometer sense element having a wide frequency response without the necessity of providing numerous through-holes in the upper plate of a torsion beam accelerometer.